Shellac will always dissolve with alcohol, meaning that if you can soak a rag in denatured alcohol and rub off the finish to the point where it changes the sheen [dull] than it's probably shellac. If it's poly I suggest staying with poly. It might also be old style [oleoresinous] varnish, depending on the age of the finish. Does it scratch easily? If it has maintained it's shine pretty well it's probably poly and i would test a spot before shellacing the whole floor.

The problem is that denatured alcohol will also disolve water based finishes and many floors are being finished now with water based topcoats. It will take a bit more elbow grease though to dull the sheen of a water based finish though with alcohol. I think it is highly unlikely though that your floors are finished with shellac unless it's an old finish. Also, why use steel wool? Just sand it.